Dendritic Morphologies of Hot-Dip Galvanized Zn-0.2 Wt Pct Al Coatings on Steel Sheets

Abstract

The dendritic morphologies of hot-dip galvanized Zn-0.2 wt pct Al coatings on steel sheets with two different surface roughness conditions were systematically examined as functions of the inclination angle and axis of the Zn basal plane with respect to the sheet surface by using electron backscattered diffraction. When the inclination axis of the basal plane was < 1 (2) over bar 10 >, the dendrite changed its morphology by following the sequence of six-fold -> eight-fold -> elongated X -> elongated X + C patterns with increasing inclination angle. When the inclination axis was < 10 (1) over bar0 >, the sequence of morphological patterns on the mirror-polished steel substrate was six-fold -> eight-fold -> two-fold -> four-fold, whereas, for the steel substrate with a rough surface, the eight-fold pattern was missing in the sequence. These sequences of morphological changes indicated the presence of a third family of preferred dendritic growth directions, in addition to the two previously known families of < 10 (1) over bar0 > and < 0001 >. The third growth direction family is proposed to be those normal to the {1 (2) over bar 11}planes. The absence of the eight-fold pattern on the rough steel surface was attributed to the surface undulation that perturbed the growth in the six weakly preferred directions toward the two strongly preferred < 10 (1) over bar0 > directions.